Making a 3D semicircle and then hollowing out a portion of it
I don't think hollowing it out is the advised way to proceed, booleans should be avoided.

Just start out with a flat mesh in the shape of the straight part of the D. Make sure it only half of it with the object center correctly positioned so we can symitrize it later

Dividide it at the desired proportions, or extrude a lower part corresponding to the desired letter width.

Correctly place your 3D cursor at the center of the shape and use the Spin operator to create half the desired curvature.

Then just let modifiers do the work for you dynamically. Add both a Mirror and a Solidify modifier to add thickness.